1. Field of the Invention
The present invention relates to a filter, a duplexer, a radio frequency module, and a communication apparatus used for wireless communication.
2. Description of Related Art
In recent years, a filter for radio frequency communication in which an antenna terminal functioning as an unbalanced port and a receiving terminal functioning as a balanced port has been required in order to prevent disturbance and improve reception sensitivity. For example, as the filter, a DMS (double-mode SAW) filter using a SAW (surface acoustic wave) device has been used.
In addition, there is a balanced-unbalanced duplexer without using the DMS filter. JP-A-2002-198765 discloses a balanced-unbalanced duplexer obtained by combining a balun (impedance conversion circuit) with an unbalanced ladder filter. As illustrated in FIG. 17, in the balanced-unbalanced duplexer, a balanced input/output receiving filter 81 is connected to a balun 71. The balanced input/output receiving filter 81 is a 4-terminal filter including two filters. In the balanced-unbalanced duplexer, the balun 71 is connected to two terminals (an input 1 and an input 2) of the 4-terminal filter. The balun 71 and a transmitting filter 61 are connected to an antenna terminal Ant.
FIG. 18 is a diagram illustrating the balun 71 and the balanced input/output receiving filter 81. The balance input/output receiving filter 81 illustrated in FIG. 18 includes two ladder filters that share parallel resonators P1 to P4 and do not have a connection point connected to the ground. This circuit is referred to as a full ladder circuit.
FIG. 19 is a diagram illustrating the balanced input/output receiving filter 81. In the balanced input/output receiving filter 81 illustrated in FIG. 19, two ladder filters 82 and 83 are symmetrically arranged. Parallel resonators L1-P1 to L1-P4 and parallel resonators L2-P1 to L2-P4 are connected to the ground, similar to an unbalanced filter. This circuit is referred to as a half ladder circuit.
In both the full ladder circuit and the half ladder circuit, there is a phase angle difference of 180° between channels 1 and 2 in a reception (Rx) band (the phases are opposite to each other). That is, these circuits are unbalanced-balanced output filters. However, when these circuits are actually used, the following problems may arise. FIG. 20A is a graph illustrating transmission characteristics between a transmitting terminal Tx and an antenna terminal Ant. and between the antenna terminal Ant. and a receiving terminal Rx of the full ladder circuit illustrated in FIG. 18. FIG. 20B is a graph illustrating isolation characteristics between the transmitting terminal Tx and the receiving terminal Rx. FIGS. 21A and 21B are graphs the illustrating transmission characteristics and isolation characteristics, as described above, of the half ladder circuit illustrated in FIG. 19. As may be seen from these graphs, in both the full ladder circuit and the half ladder circuit, the attenuation and isolation characteristics of the receiving filter in a transmission band rapidly decrease in the vicinity of a reception band and then increase thereafter. Therefore, good attenuation and isolation characteristics are not obtained in the opposite band. In general, the receiving filter requires an attenuation and isolation of about 44 to 55 dB or more in the opposite band. It is difficult for the circuit illustrated in FIGS. 18 and 19 to satisfy this performance. In addition, it is generally difficult for the full ladder circuit to remove a common mode.
In order to solve these problems, JP-A-2002-198765 discloses an aspect in which an inductance is connected to one end of each parallel resonator and the other end of the inductance is connected to the ground.